Alzheimer's Disease (AD) is a leading cause of senile dementia with over 44 million affected persons and an economic burden of over $600 billion (Mayeaux and Stern, 2012 CSH Perspect Med 2(8)). Neuropathological hallmarks of AD include amyloid plaques containing deposited Aβ peptides, neurofibrillary tangles containing hyperphosphorylated tau proteins, and in 60-90% of cases, cerebral vessel disease, including cerebral amyloid angiopathy (CAA), small vessel disease, and microvascular degeneration (Attems and Jellinger, 2014 BMC Med 12(206)).
Apolipoprotein (apo)E, which is primarily secreted in the brain from astrocytes, and also from microglia and pericytes, has been hypothesized to contribute to cerebrovascular dysfunction (Zlokovic, 2013 JAMA Neurol 70(4):440-444) and also is the principal lipid carrier within the brain, having a role in lipid metabolism (Gin et al., 2016 Clin Intery Aging 11:665-681). ApoE has been implicated in Aβ metabolism with apoE4 being detrimental, apoE3 neutral and apoE2 protective. The major routes by which Aβ is cleared from the brain involve the cerebrovasculature (Ueno et al., 2014 Curr Med Chem 21(35):4085-90), with implications for dementia (Snyder et al., 2015 Alzheimers Dement 11(6):710-7).
Cardiovascular risk factors, including type 2 diabetes mellitus (T2DM), hypertension, hypercholesterolemia, obesity and stroke increase AD risk (Duron and Hanon, 2008 Vasc Health Risk Manag 4(2):363-81). Epidemiological studies suggest that risk of AD may be attenuated by high levels of circulating high-density lipoprotein cholesterol (HDL-C), which has been associated with reduced cardiovascular disease (CVD) risk (Zuliani et al., 2010 J Gerontol A Biol Sci Med Sci 65A(5):559-564). Specifically, levels of apoA-I, the major HDL-associated protein, have been shown to positively correlate with Mini-Mental State Examination (MMSE) and Cognitive Ability Screening Instrument (CASI) scores (Shih et al., 2014 J Alzheimers Dis 41(3):855-65; Merched et al., 2000 Neurobiol Aging 21(1):27-30) and high serum HDL-cholesterol (HDL-C) levels (>55 mg/dL) in cognitively normal elderly individuals are associated with significantly reduced risk (HR 0.4) of AD, even after adjusting for APOE genotype and vascular risk factors including obesity and T2DM (Reitz et al., 2010 Arch Neurol 67(12):1491-7). In symptomatic AD patients, plasma apoA-I levels negatively correlate with hippocampal and whole brain volume as well as mean entorhinal cortical thickness (Hye et al., 2014 Alzheimers Dement 10(6):799-807), and decreased levels of serum apoA-I can discriminate AD from non-demented age-matched control subjects (Shih et al., 2014 J Alzheimers Dis 41(3):855-65). HDL and apoE have several potent vasoprotective functions including reducing inflammation, increasing vascular tone through promoting endothelial nitric oxide (NO) synthase activity, and suppressing vascular adhesion molecule expression (Stukas et al., 2014 Cell Metab 19(4):574-91; Sacre et al., 2003 FEBS Lett 540:181-187).
Animal models, such as mice genetically engineered to express human amyloid precursor protein (APP), which enables the study of progressive accumulation of Aβ and β-amyloid, have been used to study Aβ egress through cerebral vessels. There are however innate physiological differences between murine and human lipoprotein metabolism (Getz and Reardon, 2012 Arterioscler Thromb Vasc Biol 32(5):1104-15). For example, the primary circulating lipoprotein in rodents is HDL, which, due to its multiple vasoprotective functions, bestows upon mice a natural resilience to cardiovascular diseases such as atherosclerosis. By contrast, the major circulating lipoprotein in humans is low-density lipoprotein (LDL), which is mechanistically linked to vascular dysfunction and cardiovascular disease.
In vitro studies using human cells represents an alternative approach, however, most studies of the blood brain barrier (BBB) use monotypic cultures of brain endothelial cells (ECs), which do not mimic the complexity of cell-cell and/or cell-matrix interactions found in the native vessel. ECs and astrocytes, EC and smooth muscle cells (SMC) or EC and pericytes have been co-cultured, yet this is typically done under static culture conditions. A more recent study described an EC and astrocyte co-culture model using a complex flow system, but this model did not allow histology analysis or cell-extracellular matrix (ECM) interactions to be assessed (Cucullo et al., 2007 Epilepsia 48(3):505-516). A recent study using human induced pluripotent stem cells (IPSC)-derived neurons have demonstrated the formation on complex neurological organoids, but this model lacks the vasculature (Choi et al., 2014 Nature 515: 274-278.)